1. Field of the Invention
The present invention relates to a metal material consisting of Ti--Fe--Mischmetal for storing hydrogen at a high density in a stable manner.
2. Description of the Prior Art
Quite recently, a method of storing hydrogen in a metal or alloy in the form of metal hydride has been developed and this method has been applied to the storage, transportation, separation and purification of hydrogen. Ti--Fe has been developed as a typical metal alloy for use in this method of storing hydrogen. For instance, U.S. Pat. Nos. 3,508,414 and 3,516,263 disclose methods of storing hydrogen in a Ti--Fe alloy. In the hydrogen storage alloys disclosed in these patents, each has its own unique characteristics and differs from other alloys in, for example, activation, plateau property and hysteresis. Thus, various studies have been carried out with the aim of improving the characteristics of Ti--Fe alloys by the addition of other metal elements so as to provide alloys which are well adapted to other specific purposes.
The term "plateau" as used with respect to a hydrogen storage alloy in this specification refers generally to the horizontal flatness of an absorption or desorption isotherm obtained from the equilibrium hydrogen pressure-atomic ratio (H/M, H refers to the atomic weight of hydrogen and M to the atomic weight of metal element.) In case the plateau is not horizontally flat, at the time of hydrogen desorption, the hydrogen pressure is so gradually decreased that hydrogen hardly desorbs unless the desorption pressure tends to be low. Further, at the time of hydrogen absorption, with the increase in the amount of hydrogen storage, hydrogen is hardly absorbed unless the hydrogen pressure is much increased.
In the use of a hydrogen storage alloy having a plateau for storing hydrogen, it is difficult to maintain the hydrogen desorption pressure at a constant value. Further with a decrease of the desorption pressure, the amount of desorption decreases. In the use of the hydrogen storage alloy in an air conditioning system or in a heat engine of the waste heat recovery type, it is desirable that the pressure difference (.DELTA.P) between the equilibrium dissociation pressure of two different types of alloys and the equilibrium absorption pressure of the other alloy should be constant independently of the hydrogen content of the alloy (H/M). When the plateau property is horizontally flat, the hydrogen desorption and adsorption reactions of both alloys proceed smoothly since the pressure difference (.DELTA.P) is maintained constant.
In the Ti-Fe alloy, in order to activate the alloys to be reactive to hydrogen, it is required to periodically subject the alloy to elevated temperatures of higher than 400.degree. C.-vacuum and to high pressure hydrogen treatment (30-60 Kg/cm.sup.2) at room temperature for periods as long as one week.
Thus, where a Ti--Fe alloy is used for hydrogen storage or heat storage, since the alloy has to be subjected to high temperature and high pressure treatment, it follows that the container for the hydrogen storage alloy or the heat storage vessel must be made of a heat resistant and pressure resistant material. Therefore, it is necessary to use an expensive container or vessel and also to take various special precautions against the dangers of high temperature and high pressure in setting up the installation.
In addition, the necessity of carrying out repeated high temperature-high pressure treatments over an extended period of time is undesirable from the point of energy and labor costs in practical application. These drawbacks of hydrogen storage alloys have hampered their utilization in practical systems.
It has been found that this disadvantage can be overcome by replacing a part of the iron with Nb, Mn or Ti. However, although the addition of such an element improves the activation, use of Nb greatly increases the cost of the alloy while use of Mn or Ti disadvantageously results in variation of the hydrogen equilibrium dissociation pressure, thus causing the plateau property to become unsatisfactory. In addition, in the Ti--Mn alloy, in order to enhance the plateau property, the addition of other elements such as Zr, V, and Cr has been disclosed.
In general, when the alloy absorbs hydrogen, its volume expands by 10-30%. The repetitive absorption and desorption of hydrogen in the alloy is thus accompanied by repetitive expansion and contraction of the alloy with the result that the alloy is pulverized. Alloys such as FeTi-oxide, FeTiNb, FeTiMn, FeTiNbZr, etc. are known for their easy activation but they tend to be pulverized easily. The pulverized alloy will intermingle into the hydrogen gas desorbed from the alloy to cause the pipes and valves of the system to be clogged. Moreover, when it diffuses into the open air, it gives rise to environmental pollution.
U.S. Pat. No. 4,079,523 discloses that a low-oxygen Fe--Ti-Mischmetal alloy obtained by adding 0.05-1.5 weight % mischmetal (referred to as Mm hereinafter) to Fe--Ti alloy can be air-melted. According to the U.S. patent, comparison of this alloy with the conventional Fe--Ti alloy shows that while the conditions required for activation (450.degree. C. and a hydrogen pressure of 68 atmospheres) are the same as those for the conventional Fe--Ti alloy, the time required for activation is 38 hours as against 111 hours for the conventional alloy.
Thus, it has been known to improve the characteristics of the alloy by the addition of metal elements. However, from the practical aspects of production cost, plateau property, hydrogen storage capacity, service life etc., there still remain various problems to be solved.